Electrical Discharge Machining is a process by which conductive particles are removed from the surface of a positively charged workpiece by a series of discharges emanating from a negatively charged electrode. The electrical discharges or sparks create micro-craters on the workpiece by removing material along the cutting path through melting and vaporization. The particles are then washed away by a continuously flushing dielectric fluid. EDM is typically used to create very small and accurate holes having generally straight walls. One common application for EDM is in the fabrication of fuel injector nozzles having one or more injection orifices.
It has recently been recognized that a reverse taper in the orifice of a fuel injector tip (i.e., a generally conically-shaped hole originating from a larger diameter at an internal surface the injector tip and terminating at a smaller diameter at an external surface of the injector tip) improves injection flow characteristics. Although EDM has been used to produce injection orifices in the past, the orifices produced by EDM were limited to straight walls (i.e., walls without significant taper). Therefore, a new process was required to produce the desired reverse taper.
One EDM process utilized to produce reverse tapered holes is described in an article by Diver et al. entitled “Micro-EDM drilling of tapered holes for industrial applications” published in the Journal of Materials Processing Technology Vol. 149, pages 296-303 (2004). This article describes a process in which an electrode is presented to a workpiece at an angle parallel with an angle of a desired taper. As the electrode is charged and fed toward the workpiece, the electrode is rotated about a vertical axis such that a spiraling cutting trajectory is formed. That is, as the electrode is rotated, its angle allows the electrode to cut an internal annular swath of the workpiece. As the rotated electrode is advanced toward the workpiece, a cone shape is achieved with the increasing depth and diameter of cut.
Although the process outlined in the above-identified article may be capable of producing the required taper, it may be complex, expensive, time consuming, and insufficiently accurate. Specifically, additional components must be added to the typical EDM apparatus to produce the angled rotation of the electrode. Similarly, additional control mechanisms must be utilized to regulate the motion of the rotating components. These extra components and control mechanisms increase the complexity and cost of the EDM apparatus. In addition, because only one annular segment of the taper's diameter is being cut at a time (i.e., the electrode must be rotated 360° to cut an entire periphery of the taper at a given depth), the time require to produce the entire tapered cut may be significantly more than the time required to produce a straight cut where the entire periphery at a single depth is simultaneously cut. Further, the process outlined above may require a greater electrode length. The greater electrode length, combined with the cantilevered angle, may allow for greater vibration in the electrode that could produce inconsistencies in the taper.
The present disclosure is directed to overcoming one or more of the shortcomings set forth above.